I suspect many bridges were lightweight girder on supports and probably then sleepered, USA style. The concrete(?) supports have lasted longer than the railways, so maybe a bit overbuild, then if they wee meant to cope with potential of being blown up then it makes sense. The picture of what looks like a concrete arched bridge is interesting, typical in some ways of later French builds, maybe 1920s. As it has been converted to a road, can't see what the track was like.